This invention relates to the preparation of sulphides. The invention has application in the synthesis of phosphors, pigments, ceramics and optoelectronic materials. The phosphors may be used for photoluminescence, high and low voltage cathodoluminescence, and AC or DC electroluminescence. The phosphors can also be used in x-ray applications and for thermoluminescent storage.
Known methods of preparing sulphides include:
i) the reaction between gaseous hydrogen sulphide and a solution containing the appropriate cation, and
ii) the thermal and/or catalytic decomposition of an organo-sulphur compound in a solution containing the desired cation.
Typical of the reagents employed in the second method are thiourea and thioacetamide. This method employs controlled release of the anionic species uniformly throughout the reaction container and is commonly known as homogeneous precipitation.
Both methods have some disadvantageous features. In the first method the use of highly toxic gaseous reagents can be extremely hazardous. In addition, it is believed that this method leads to the precipitation of non-stoichiometric materials having an irregular, plate-like morphology. As to the second method, the principal disadvantage of most organo-sulphur compounds is their high cost. A further disadvantage is that the precipitated product is usually contaminated with the organic starting compound.
Recently there has been published in International Patent Publication No. WO 98/55395 a method of preparing sulphides which comprises the steps of dissolving sulphur in hydrogen hydrate and then combining the resulting solution with a solution of an appropriate cation to precipitate the corresponding sulphide.
It is an object of the invention to provide an alternative method of preparing sulphides which does not generate toxic fumes.
Accordingly the present invention in one aspect comprises a method of preparing sulphides in which a dithionite compound is combined with an aqueous solution of an appropriate cation to precipitate the corresponding sulphide.
The dithionite compound may be a metal or other cationic dithionite compound. Preferably the combination of the dithionite compound with an appropriate cation takes place in an aqueous solution which is alkaline.
In carrying out the invention the dithionite compound may be produced as an intermediary in the aqueous thermal decomposition of formamidinesulfinic acid. This substance, which is also known as thiourea dioxide, has the chemical formula H2NC(NH)SO2H.
Thiourea dioxide may therefore be used as the starting material and according to the invention in another aspect a method of preparing sulphides comprises combining thiourea dioxide with a solution of an appropriate cation to precipitate the corresponding sulphide. In this method a dithionite dianion is produced as an intermediary during the reaction of thiourea dioxide with the cation.
Preferably the precipitate is subsequently fired. The solution may comprise a mixture of appropriate cations. Examples of appropriate cations are zinc and copper.